1. Field of the Invention
This invention relates to an image forming apparatus and, more specifically, to an image forming apparatus such as a multicolor printer apparatus or a full-color copying apparatus utilizing an electrophotographic process.
2. Description of the Related Art
In the copying (printer) apparatus, it is known that the surface potential applied to the photoconductor depends on its environmental temperature, and moisture and on the number of accumulated copying sheets.
The surface potential of the photoconductor is particularly important in a full-color process. A variation in an amount of toner degrades the color balance of a formed image. Consequently, the intensity of the developing voltage applied to each developing device for every process has to be set to a fixed value, irrespective of the number of times the process is repeated.
Therefore, in the copying (printer) apparatus, a given allowable margin is provided to image forming materials and an image forming process itself, and image stabilization is attained by maintenance within this allowable margin.
However, the allowable margin to be provided to the image forming materials and image forming process itself is limited, and the maintenance required much labor and cost. Furthermore, the image density drift cycle is shorter than a maintenance cycle, and a stable image density cannot always be obtained by only the maintenance.
A method has been so far proposed to keep the surface potential applied to the photoconductor to a fixed value.
The method is that the surface potentials applied to a photoconductor are measured directly after charge is supplied from the charging device. A decay curve is obtained by the measured surface potentials to control the intensity of charge supplied from the charging device to the photoconductor. This method is disclosed in Published Unexamined Japanese Patent Applications Nos. 61-238070 and 2-77766.
However, the above methods have the following drawbacks.
In the method of Published Unexamined Japanese Patent Application No. 61-238070, since the sensors for sensing the surface potential are very expensive, production costs greatly increase. If a sensor is provided for each of the developing devices, the production costs excessively increase. Further, the apparatus has to increase in size because it includes plural sensors. If the apparatus includes a plurality of developing devices the expensive sensors have to be protected from movement of the developing devices.
In the method of Published Unexamined Japanese Patent Application No. 2-77766, the photoconductor is intermittent and, in this case, no image can be formed during the measurement of potentials.